It is necessary in analytical systems to have fittings that create leak-tight seals. In such analytical systems, it is also desirous to have fittings which are inert relative to the sample components, which provide a flow path without inducing turbulence or mixing, and which adds minimal volume to the system.
Fitting designs which best address the aspect of added volume allow the tube ends which pass through such fittings to butt directly to each other or have bores which match the tubing bore, leaving no dead or unswept volume. Such fittings are called zero dead volume fittings. In analytical testing apparati dead volume is to be avoided as it reduces efficiencies of the test equipment, including gas columns.
One of the most common types of such zero dead volume fittings is a compression fitting. A zero dead volume compression fitting consists of a fitting having a female fitting detail, tubing, a ferrule loosely riding on the tubing, and a male nut, also riding on the tubing. In operation the tubing is placed into the fining until the tubing end passes through the inner bore of the fitting, and seats correctly at the bottom of the fitting detail, the ferrule is then slid along the tubing until, it engages the mating walls of the fitting, and the nut is threadedly engaged so as to be retained in place and to deform the ferrule against the mating wall of the fitting detail, causing the ferrule to apply pressure to the tubing and force such tubing against the bottom of the fitting detail.
Basic problems with such system are inherent in the type and number of parts, namely the fitting, the ferrule, the tubing and nut. The fitting and ferrule need be sufficiently sized so that the ferrule can be deformed against the fitting to provide a seal, which often provides some volume at the interface of these components. The ferrule in these systems necessarily must be deformable to provide a seal, but the force necessary to do so may be exceeded and thus overdrive the tubing into the fitting and interfere with operation of the associated valve. This problem is acerbated in a multi-ported valve having a plurality of fitting bodies associated with it and the overtightening into one or more of the detail associated with a port. Moreover, these systems must sustain the associated high pressures, must not deform the associated fitting, and must not have any extraneous volume creating an undesirable mixing chamber.